You do not have to take 2 RMDs in one year. You may delay your first RMD - so in your husband's case you can push his 2017 RMD to 2018, but then he would have to satisfy his 2018 RMD in in 2018 as well. Most of the time it makes sense to take them in separate years unless you think your income is going to drop drastically in 2018.
So if he is taking an RMD in 2017, you look at the Dec. 31, 2016 balance of each IRA and divide it by the IRS divisor. If he is 70.5 in 2017 his divisor is 27.4. So if he has $100,000 at the end of the year in 2016, his RMD for 2017 will be $3650. If you have multiple IRAs, there is no problem taking the entire amount from 1 IRA, as long as you satisfy it.